James Polk, the 11th President of the United States, rarely merits a mention in the popular history of his country. The original 'dark horse', he rose to defeat the charismatic Henry Clay in the election of 1844 by the narrowest of margins. Polk had made a campaign promise that he would serve only for four years and he was true to his word. Yet what occurred during his time in office was arguably as significant as anything experienced during the administrations of Washington, Lincoln or Franklin Roosevelt.

The America that Polk presided over was a confident nation. Independence from Britain had been secured just over half a century earlier and the Louisiana Purchase from the French in 1803 revealed a country growing in stature and territory. Providence, it seemed, was smiling on the United States.

This was a theme elaborated on by contemporary writers: 'The great experiment of liberty,' explained New York Morning News editor John O'Sullivan, knew no bounds. It was America's 'manifest destiny' to expand. All eyes naturally turned West. Herman Melville argued: 'America can hardly be said to have any western bound but the ocean that washes Asia.' Walt Whitman looked to California and New Mexico and wondered: 'How long a time will elapse before they shine as two new stars in our mighty firmament?'

The answer was: not long. Blood and Thunder traces the path that US forces trod to fulfil this yearning. For all the idealist and idyllic lines of Whitman and Melville, the West would not be secured without conflict with the Mexicans, who also had designs on the adjoining territories, and the native Indians, who had lived there for several hundred years.

In 1845, Texas was annexed and a year later, Polk found a casus belli to declare war on Mexico after it turned down his offer to buy the two states. Three years later, with the treaty of Guadalupe Hidalgo, Mexico had been halved in size and California, Nevada and Utah, as well as parts of New Mexico and Arizona, added to the USA.

The problems were only beginning. As Hampton Sides explains, the Americans were confronted with the tensions that occupying forces have faced down the ages: 'The longer [they] stayed, the more the people resented them - not only for the central fact of their conquest, but for the thousand little insults and daily humiliations committed by an uncouth foreigner who considered himself, in every possible way, superior.'

Their predicament was doubly difficult since they not only had to contend with attacks from those over whom they claimed dominion, but also to resolve the bitter hatreds between the Mexicans and the Indians, most notably the Navajo tribe.

The Navajos proved the more difficult to accommodate. It was not that they did not want peace with the invaders - they were curious to meet them. Rather, they had no understanding of the terms that the new arrivals wished to impose. Agreements would be reached, only for the Navajos blithely to revert to stealing horses and cattle from the locals.

Frustrated, the Americans adopted a scorched-earth policy to drive them into reservations, which they judged the best means to control and 'civilise' them. These proved to be as deleterious - contact with American soldiers ensured that diseases, such as syphilis, spread quickly - and as popular with the indigenous people as strategic hamlets were with the Vietnamese a hundred years later.

Sides's lone hero in all this is Kit Carson. A frontiersman who became a myth in his own lifetime, Carson was involved at every key stage in the American advance. His skills as a tracker singled him out first as a guide for the early expeditions through California, second as a messenger and, finally, as a military leader in New Mexico. A reluctant warrior, he lacked his countrymen's instinctive antipathy towards the Mexicans and Indians. His motivation in abetting the US army was to deliver stability to the region.

Romanticism attaches itself to Carson alone. Scalping, lynching, rape and other forms of butchery were practised by all factions. Sides moves adeptly between the blood and gore of battle and the intrigue driving American strategy, if it can be called such. If his narrative sometimes appears haphazard and messy, it is because the conquest of the West was just that. This was manifest destiny in its strangest and bloodiest form.